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COWLEY, T.E.; CUNNINGHAM, P.L.; JOUBERT, D.F.
HENSCHEL, JOH R.; MTULENI, VILHO; PALLETT, JOHN; SEELY, MARY K.
A vegetation monitoring programme has been carried out at Rössing Uranium in the Khan River since 1988 to investigate the effect of groundwater extraction on the riparian vegetation. The survey methods and monitoring results are described in this article.
One of the main conclusions is that most large woody species can adapt to a wide range of water table fluctuations. Faidherbia albida was found to be more sensitive than Acacia erioloba to rapid water level declines and to deep water levels around 15-20 m below surface.
vegetation monitoring, impact of groundwater extraction, Khan River, Rössing Uranium, Namibia
Rössing Uranium is located in the Namib Desert approximately 65 km inland of Swakopmund (Figure 1). The operation consists of an open pit and an acid-leach uranium plant. The area experiences low and erratic rainfall, high temperatures and evaporation rates. Due to the desert environment and the nature and size of the process, considerable attention is paid to sustainable management of the available water resources.
From the onset of operation in 1976, the mine has used brackish water from the Khan River for industrial purposes to reduce the consumption of fresh water. The Khan is an ephemeral river and surface runoff usually occurs only after heavy rainfalls in the upper catchment area. For the remainder of the time, groundwater flows below the surface of the riverbed within deep layers of sand and gravel.
The Department of Water Affairs (DWA) controls the extraction from the river by means of an abstraction permit. The original permit allowed a quota of 0.87 million cubic metres per annum (Mm³/a), which was defined as the sustainable yield of the aquifer from which the Rössing wellfield draws (Dziembowski 1970). In 1988, the mine extended the well field into the "upstream compartment" and applied for an increase to 1.0 Mm³/a, in order to maintain high standards of dust suppression.
A hydrogeological study indicated that the water table would eventually be lowered to 10 metres below surface, but groundwater reserves at a depth of 10-20 metres below surface would still be available to sustain the vegetation (Groundwater Consulting Services 1989). Concern was expressed that the trees would not be able to adapt to the falling water table and could therefore be damaged if water was extracted too rapidly (Ashton 1988).
Lower water levels could also affect soil conditions and other components of the eco-system. The DWA agreed to grant the increased quota on condition that the riverine vegetation was monitored. The monitoring results would be submitted to the DWA to provide an early warning system and the pumping rate would be revised if abstraction was indeed found to affect the trees.
A distinction had to be made however, as to whether the effects are localized and thus due to water abstraction, or due to more widespread regional climatic effects. Therefore the survey would have to be conducted over a wider area, if signs of water stress were noted at the mine (Ashton 1988). The vegetation monitoring programme started in 1988, some 12 years after the start of mining, and continues to the present. No baseline study was carried out in the Khan River prior to the establishment of the mine.
3. Composition of the vegetation
The vegetation of the area around Rössing mine forms part of the transitional zone between the Central Namib and Semi-Desert Savannah floristic regions (Giess 1971). The savannah elements of the vegetation are usually located along riverbeds and watercourses while the semi-desert and desert forms occur away from the drainage lines.
The vegetation in the Khan River consists mainly of small, scattered groups and individuals of indigenous drought-tolerant perennial species. These are mostly larger woody shrubs and trees that colonize sandbanks in the riverbed and flood terraces along the banks. Alien species like Prosopis sp. and Nicotiana glauca are quite common and annual species can become abundant after flood events. Ashton (1988) conducted an initial survey of the Khan River near Rössing Mine and found most of the trees and shrubs alive and in good condition, although several examples of standing-dead acacias were seen. Virtually all of the species were present as adult plants and very few seedlings were present.
These seedlings seemed to be of the same age, possibly having germinated after the exceptional floods of 1985. 4. Design of the monitoring programme Six monitoring sites (transects 1-6) were chosen over a distance of 22 km. The monitored area starts 5 km upstream of the mine, extends for 6 km along the mine frontage and ends 11 km downstream (Figure 2).
The presence of about 10 trees within an area of 1 ha was the main criterion in deciding whether or not a site was suitable. Unequal spacing between the transects reflects the scarcity of suitable groups of trees. Each monitoring site is located on a raised river terrace on the inner face of a river bend where the trees are largely protected from flood scouring.
Because two new production boreholes were installed close to transect 1 in 1993, transect 0 was established further upstream at a site thought to be unaffected by abstraction. However, monitoring boreholes drilled in 1995 showed that pumping had in fact lowered the water level. Transect KEM16, 6 km upstream of transect 1, was therefore added in 1996.
Table 1 shows the numbers and species of trees monitored at each transect. The trees were numbered and marked with metal tags. A base point at each transect and the position of each monitored tree was surveyed. Monitoring is carried out every six months at the start and end of the growing season. The survey consists of fixed-point photographs, measurements and observations. Measurements include the height and girth of the trees or main stems.
Heights are measured using a theodolite and reported as the difference between the base of the trunk and the tip of the tallest branch visible from the instrument position. The girth is measured with a tape at a height of 1.2 m above ground level, normally where the metal tag is attached. Observations are made of the general condition, presence of leaves, flowers and fruit.
Estimates of leaf coverage, leaf condition, dead branches and abundance of seeds are recorded in a semi-quantitative manner. Photographs are taken from a base point at fixed angles. They are marked with the relevant tree numbers and survey date and filed in chronological order. The files and evaluation reports are sent to the DWA for review each year. The overall condition of a tree is classified according to the observed parameters of dry branches, leaf coverage, presence of flowers and seeds.
To show this graphically, numbers were assigned to the descriptions as follows: Excellent condition = 4, Good = 3, Reasonable = 2, Poor = 1 and Dead = 0. Graphs of condition over time were prepared by calculating the average condition of all trees at a specific transect for each year. These will be discussed in 5.2. [...]